Introduction

The SII Formula

In its most basic form, the SII is the ratio of the cost of preventable crashes
that have occurred at a location to the cost of constructing the
proposed improvement. The SII incorporates adjustments to provide
additional benefit for:

Obtaining SII Data

Before calculating the SII, the “Proposed Corrective Action”
must be translated into “work codes.” The HSIP Work Codes Table
(contained in Section 7 of this manual) provides the work codes
that correspond to various descriptions of work. The table also
provides associated definitions, reduction factors, service lives,
applicable maintenance cost, and preventable crash codes (see following explanation).

The data necessary to calculate each project’s SII can be
obtained from the sources shown in the following table.

NOTE: The reduction factor represents the percentage reduction
in crash costs or severity that can be expected as a result of the improvement.

From the Highway Safety Improvement
Program (HSIP) Work Codes Table (contained in Section 7 of this
manual).

NOTE: If the project is represented by more than one work
code, Texas Department of Transportation (TxDOT) Traffic Operations Division
(TRF) program administrators derive a composite reduction factor.

F — Number
of fatal and incapacitating injury crashes

I — Number of non-incapacitating injury
crashes

The HSIP Work Codes Table shows
“Preventable Crash” codes. Preventable crashes are those with defined
characteristics that may be affected by the proposed improvement
as described by the work code. The codes correspond to numeric codes
assigned in the Crash Records Information System (CRIS) to the indicated
variable. Information is collected from the peace officer’s crash
report and converted into a coded format. The Preventable Crash
Decoding Table (Section 8 of this manual) can be used to interpret
the codes and determine the number of each type of crash. Three
years of preventable crash data are used. The program call specifies
the years used.

Cf — Cost
of a fatal or incapacitating injury crash

Ci — Cost of a non-incapacitating injury
crash

The average cost of each type
of crash is based on the comprehensive cost figures provided by
the National Safety Council. The program call provides the cost
figures used each year.

L — Project
service life

From the HSIP Work Codes Table
found in Section 7of this manual.

NOTE: If the project is represented by more than one work
code, TRF program administrators base the project service life on
the primary work.

SII Results

A project with an SII greater than or equal to 1.0 is considered
cost effective. Projects with an SII of less than 1.0 will not be
considered for funding.

NOTE: The SII does not establish the need or lack of a need
for a project. The SII formula compares costs of preventable crashes
to costs of construction; it provides no evaluation of the appropriateness
of the type of construction.

The SII was designed as a comparison device for project prioritization
and should not be used as a measure for independent
projects.